\documentclass[border=5mm]{standalone}
\usepackage{luamplib}
\begin{document}
\mplibtextextlabel{enable}
\begin{mplibcode}
beginfig(1);
pair P, A, B, C, A', B', C', R, S, T;
P = 200 dir 102;
A = 100 dir 159; B = origin; C = 90 dir 42;
A' = 3/8[P, A]; % the factors should not
B' = 1/2[P, B]; % be the same!
C' = 5/8[P, C];
R = whatever [A, B] = whatever [A', B'];
S = whatever [B, C] = whatever [B', C'];
T = whatever [C, A] = whatever [C', A'];
path t[];
t1 = A -- B -- C -- cycle;
t2 = A' -- B' -- C' -- cycle;
fill t1 withcolor 7/8[red, white];
fill t2 withcolor 7/8[blue, white];
draw t1 withcolor 1/2 white;
draw t2 withcolor 1/2 white;